The current energy policies around the world have encouraged the development of renewable, clean and sustainable energy sources in an effort to reduce greenhouse emissions. The conversion of sustainable biomass for the production of energy and high value chemicals has been proposed as an enabling technology and received a great deal of interest for decades. From a technical stand point, chemicals extracted from biomass generate less waste than their oil-based counterparts. Thereby, various processes such as cellulosic plants, algae, triglyceride plants, and rubber plants have been suggested as feedstocks to produce biofuels (diesel, gasoline or ethanol) from biomass.
Liquid petroleum fuels are responsible for more than 93% of all transportation energy consumption in the United States, and volatility in gas prices, reduced reserves and greenhouse gas emissions have led to a rapidly growing interest in biofuels. The conversion of biomass to drop-in transportation fuels offers the benefit of providing a renewable replacement for hydrocarbon fuels currently used in a vital subset of transportation (i.e., heavy transport applications). It has also been proposed to be a method of reducing overall CO2 emissions by closing the carbon cycle. A substantial disadvantage to the use of biomass, however, is the competition with resources for food production, and thus recent research has focused on converting residual bio-waste into transportation fuels.
Unfortunately, biomass stores solar energy in the form of C—H and C—C bonds along with energy-neutral C—O and O—H bonds. To harness the energy contained it biomass, the C—O and O—H bonds should be removed while preserving the C—H and C—C bonds with maximum overall energy efficiency. Three major approaches to the conversion of biomass are currently available: gasification, pyrolysis, and hydrolysis. Gasification involves the complete breakdown of the carbohydrate biomass into CO and H2 by dissociating all the C—C and C—H bonds and most O—H bonds. The C—C and C—H bonds are then reformed via Fischer-Tropsch synthesis, resulting in a loss of 50-60% of the heat content in the biomass. Pyrolysis is a somewhat less energy intensive process, during which C—O and O—H bonds are dissociated. However, some of the C—C and C—H bonds in the biomass are also broken during pyrolysis; this leads to the semi-selective production of liquid fuels and a loss of about 45% loss of the heat from the biomass. By comparison, hydrolysis provides higher energy efficiency due its inherently milder and selective conversion. However, a series of lengthy and complicated reaction steps are typically necessary generate biofuels from biomass, because biomass is mainly composed of cellulose and lignin, both polymerized sugars. First, the cellulose biomass must undergo complete C—O bond dissociation for de-polymerization into glucoses. Then, the glucoses must be further dehydrated or isomerized to a wide spectrum of platform chemicals, among which 2-methylfuran (2MF) and butanal have recently attracted interests.
Recent research by Corma et al. has demonstrated a promising multi-step technique for producing long-chain hydrocarbons from 2-methylfuran (2MF) and butanal with high activity. First, Amberlyst-15 is used as an acid catalyst in a liquid-phase batch reaction to hydroalkylate 2MF with butanal into 1,1-bisylvylalkanes, which are then separated from 2MF, butanal, and water (side product). This intermediate product is then hydrodeoxygenated over Pt in a separate flow reactor to produce long-chain hydrocarbons. The process minimizes unnecessary C—C and C—H bond dissociation, and thus has high energy- and carbon-efficiency. However, the non-continuous combination of a batch reactor with a plug flow reactor reduces the throughput of the approach and makes scale-up challenging. Furthermore, extra energy and time are required for the separation of 1,1-bisylvylalkanes from 2MF and water.
Combining these process steps into a single, continuous reaction procedure would increase not only the energy efficiency, but also the rate of production towards long-chain hydrocarbons.